The major goal of the proposed studies in Research Project I is to develop practical methods that will improve the safety and efficacy of live attenuated SIV vaccines. The proposed experiments are also designed to allow an increased understanding of the mechanisms of protective immunity. The most highly attenuated, multiply-deleted strains will be used for many of the studies in order to help ensure long term safety. Whether the protective effects of very highly attenuated SIV strains can be boosted by soluble antigens or recombinant poxvirus will be determined. Increased neutralizing antibody responses resulting from the absence of specific sites for N-linked glycosylation will be tested for increased protective capacity. Evidence for a contribution of antibodies will be investigated in passive transfer experiments. Whether a match of gag-pol sequences, env sequences or both is important for optimal protection will be determined. Attempts to increased the breadth of protection by inclusion of multiple sequence variants in the vaccine will be investigated. Where SIV can compensate for the loss of its nef gene will be investigated and, if so, the mechanisms will be defined. These experiments are expected to bring the live attenuated vaccine approach closer to general acceptance for small scale safety testing in selected human volunteers.